1. Field of the Invention
The present invention relates to a motor driving circuit, and more particularly, to a motor driving circuit for driving a motor while reducing power consumption.
2. Description of the Prior Art
With the progress of computer technology, 3C products are increasingly becoming human-based and multi-functional. However, with enhanced computing ability also comes higher and higher heat generated by a computing chip. Thus, means of dissipating heat are garnering attention in the market. Present methods for dissipating heat generally use a heat-dissipating fan. However, the heat-dissipating fan generates a considerable amount of noise.
Please refer to FIG. 1, which is a schematic diagram of a prior art driving circuit 10 for driving a fan motor 12. As shown in FIG. 1, the driving circuit 10 is coupled between an input voltage source VIN and the fan motor 12, and comprises a control unit 100, a resistor 102 and a power switching device 104. The resistor 102 and the power switching device 104 are coupled in parallel, and a voltage across both is VIN1. The control unit 100 is coupled to a base of the power switching device 104, and is utilized for choosing a conducting route either through the resistor 102 or through the power switching device 104, so as to adjust the voltage VIN1. When the voltage VIN1 is adjusted, a voltage VIN2 across the fan motor 12 is correspondingly adjusted, thereby adjusting the speed of the fan motor 12. In other words, when the control unit 100 conducts the power switching device 104, the voltage VIN1 is reduced, causing the voltage VIN2 across the fan motor 12 to become higher and the speed of the fan motor 12 to become faster in order to dissipate more heat. When the control unit 100 turns off the power switching device 104, the conducting route will be through the resistor 102. In such a condition, the voltage VIN1 is higher, causing the voltage VIN2 across the fan motor 12 to become lower and the speed of the fan motor 12 to become slower in order to reduce noise generated by the fan.
Though the driving circuit 10 can reduce noise generated by the fan, there are still some disadvantages. For example, the driving circuit 10 utilizes the resistor 102 to lower the voltage VIN2, but the amount by which the voltage VIN2 is lowered will vary with a load current. When the voltage VIN2 is low, the load current is smaller, and therefore the amount by which the voltage VIN2 is lowered is below an expected value. Thus, when the voltage VIN2 is low, the ability to adjust the speed of the fan motor 12 is limited by the load current. Though the load current is greater when the voltage VIN2 is high, the speed of the fan motor 12 cannot be reduced linearly. Besides, whether or not the current flows through the resistor 102 or the power switching device 104, a lot of power is consumed, which lowers power efficiency, and makes realization of the driving circuit 10 with integrated circuits become difficult.